To be honest, until Greenwood plays through the middle as a bona fide starter, it's impossible to rate him against any player who had all the pressure, attention and hopes on them.
Owen was the main man/boy for club and country almost straight out the gate. Rooney had far more than that as he was immediately being compared to the greatest English player of all time and was expected, on talent alone, to become the de facto best player England had ever seen, which is obviously a different stratosphere to the other two.
Greenwood is, it's pretty clear, a few years off maturation. He's a devastating peripheral entity at the moment who doesn't have it in him as of yet to impose his will on a game or be ever-present in it, which is the key difference between he and either Owen or Rooney when they came to prominence. There wasn't a defender or backline on the planet you didn't fancy Owen against. Rooney's nemesis as a teen was his temperament and emotions, not the defenders or plans put in place to stop him. In other words, absolutely irrespective of the opposition, stage or tactical plans, both Owen and Rooney hit heights where they were the best forwards on the pitch, outright with no decoys or subterfuge. Greenwood isn't there yet and has a long way, as of now, to reach those heights.
Owen was not just about pace, and if he is being presented that way it would be disingenuous. What Owen had was breathtaking speed, for sure, but also a level of cunning, composure and instinct in and around the box that made him lethal. Dinked lobs over keepers; headers at the near post after cutting across his marker due to his positioning; one-touch finishing and just a clarity of purpose and understanding of what to do and when to do it as and when the opportunity arose. His pace was supplemental (an understatement) but it was just one part of the package that made him one of the hottest strikers on the planet in his pre-injury days. He was a line-leading, superstar striker as early as his managers would allow with next to no protection or ushering into the men's game because he instinctively knew what to do and played with such guile and elusive tendencies that he didn't have to worry about getting smashed etc. A Michael Owen who stayed injury free would've got to Greaves' status, imo.
Greenwood has the kind of talent and longstanding ability where practically everyone is tipping him to be a world class player in a year or two. He also clearly has an upward trajectory that tells you, injuries and focus permitting, we've not seen anything like the player he will become other than knowing it'll be a forward with sublime striking technique. Greenwood is the type of player it won't be a revelation to state he can surpass Owen's best in the next 5 years. With regard to Rooney, I'm not so sure as he was given absolutely everything to be the best English player of all time and even for people who believe he failed to live up to his potential, the fact he is England and United's standalone top scorer despite not being an outright #9 tells you the levels he operated on. If Greenwood can hit those heights, he's in the conversation with Charlton, Greaves, Finney, Matthews, Edwards, Rooney et al as the best of the best a nation has ever had to offer. Can't say he is default for that kind of categorisation like Rooney was straight out of the gate, which is no slight on Greenwood as Rooney is an outlier in that sense to which I've never seen any other English talent immediately thrust into that level of regard before or since.
The tl: dr of this is, as far as talent goes:
1. Rooney
2. Owen=Greenwood
World impact to date of Greenwood's current age:
1. Rooney - star of an international tournament up to the point of injury.
2. Owen - world prominence on the grandest stage there is. Absolutely lethal as a teenager.
3. Greenwood - clearly an elite talent, but still peripheral in a number of games.
Potential:
1. Rooney - the only English talent compared to Charlton and Di Stefano and noted as White Pelé in part for a bit of a giggle, but also because of how overarching his skillset was.
2. Owen =/= Greenwood - Owen's pre-injury path and ability was exceptional. Greenwood has it in him to be one of the players of his generation. Ask again in 36 months and it'll be evident whether we're looking at a player for this era, or just outright one of the greatest of all time.